1. Field of the Invention
The present invention generally relates to an image forming apparatus which forms an image on a surface of a photosensitive body through an optical scanning device, and, in particular, to an image forming apparatus by which, even employing a device having a low resolution, an apparatus having an apparently high resolution is provided.
2. Description of the Related Art
Recently, improvement in resolution of an image forming apparatus has been demanded as in an LBP (laser beam printer) and so forth.
In order to render high resolution without increasing the printing speed of an LBP, {circle around (1)} the speed of rotation of a deflector such as a polygon scanner may be increased, {circle around (2)} the number of light sources may be increased and thereby scanning is rendered with a plurality of scan lines at once, or the like.
However, {circle around (1)} when the speed of rotation of a deflector such as a polygon scanner is increased, not only the polygon scanner itself becomes expensive, but also the durability thereof becomes degraded, the noise therefrom increases, the power consumption thereof increases, and so forth.
Further, when {circle around (2)} the number of light sources is increased and thereby scanning is rendered with a plurality of scan lines at once, a configuration for composing a plurality of beams becomes complicated, the number of parts/components increases, thereby costs increasing, and assembling accuracy of the mounted parts/components cannot be maintained because of complexity of the device.
Further, improvement of resolution only in a main scan direction (the same as a deflection direction of a deflector) can be rendered by increasing a modulation frequency of a light source. However, by this method, it is not possible to improve resolution in a sub-scan direction.
As a method of solving the above-mentioned problems, and improving resolution in a main scan direction and also in a sub-scan direction, TrueRes technology is proposed in a document xe2x80x98technology of improving resolution of page printerxe2x80x94TrueRes Interface January/February, 1996xe2x80x99. This method has also been put into practice.
The TrueRes technology is to utilize a characteristic in that a toner-adhering amount of toner adhering to a surface of a photosensitive body with respect to an exposure time for which the photosensitive body is exposed by a beam is determined by an accumulation of the exposure time (exposure distribution).
FIG. 1 shows a characteristic of an exposure time and a toner-adhering amount.
In FIG. 1, in a range in that the exposure time xe2x80x98txe2x80x99 is not longer than t3 (txe2x89xa6t3), a reaction critical value has not been exceeded, and the toner-adhering amount TM is substantially zero.
In a range in that the exposure time falls between t3 and t4 (t3 less than t less than t4), the reaction critical value has been exceeded. Then, in a range in that the exposure time t is equal to or longer than t4 (txe2x89xa7t4), the toner-adhering amount TM becomes TM0, and is saturated.
Because the toner-adhering amount is thus determined by accumulation of the exposure time, the toner-adhering amount becomes TM0 by four times of exposure each rendered for the interval t1, and, this is the same as the toner-adhering amount rendered by once of exposure rendered for the interval t4.
Similarly, in a case where exposure is rendered for the interval t2, the toner-adhering amount reaches TM0 by twice of the exposure.
FIG. 2 shows an image of improving resolution employing the TrueRes technology.
In FIG. 2, G11 through G13 represent dots formed on a scan line L1, G21 through G23 represent dots formed on a scan line L2, and, for each of them, exposure is rendered for an interval equal to or longer than t4 in the characteristic shown in FIG. 1, and, thereby, the toner-adhering amount is TM0.
Further, dots GA formed between adjacent dots can be formed through twice of exposure each rendered for the interval t2 in the characteristic shown in FIG. 1 on the positions of these adjacent dots, respectively.
For example, a new dot GA can be formed between the dots G11 and G12 on the main scan line L1 in the main scan direction, and another new dot GA can be formed between the dot G11 on the main scan line L1 and the dot G21 on the main scan line L2 in the sub-scan direction.
Thus, by employing the TrueRes technology, it is possible to improve resolution in the main scan direction and also in the sub-scan direction.
Further, in an image forming apparatus in the related art, as disclosed in Japanese Laid-Open Patent Application No. 10-250144 by the present applicant, by employing the TrueRes technology, employing a plurality of LED (light emitting diode) arrays in a light source thereof, configuring the respective LED arrays in the main scan direction of a photosensitive body, and controlling turning on/off of each diode, new dots are formed between adjacent normal dots in the main scan direction and in the sub-scan direction.
In the image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 10-250144, the diameters of newly formed dots can be made to approximate those of the normal dots.
In the image forming apparatus disclosed in Japanese Laid-Open Patent Application No. 10-250144 employing the TrueRes technology, resolution of newly formed dots is determined by exposure distribution, and, also, the exposure distribution is mainly determined by a static beam-spot diameter and intervals between scan lines in the sub-scan direction. Accordingly, the resolution is not improved unless the static beam-spot diameter and intervals between scan lines in the sub-scan direction are properly set.
The present invention has been devised in order to solve such a problem, and an object of the present invention is to provide an image forming apparatus in which the static beam-spot diameter and intervals between scan lines in the sub-scan direction are properly set, and, thereby, resolution in the sub-scan direction is improved.
Another object of the present invention is to set a condition of improving resolution in the main scan direction.
According to the present invention,
a dot is formed at a center between adjacent light fluxes as a result of the adjacent light fluxes being overlapped with one another in a sub-scan direction, and
a ratio of a static beam-spot diameter Ws in the sub-scan direction on the surface of a photosensitive body defined by 1/e2 of the maximum value in the exposure distribution of the beam spot (the diameter of the area of the beam spot through which the exposure intensity is not lower than 1/e2 of the maximum value thereof, also hereinafter) to an interval L (in a center-to-center basis, also hereinafter) between adjacent scan lines satisfies the following formula:
1.2 less than Ws/L less than 4.5 
Thereby, it is possible to easily form a dot at the center between adjacent scan lines, to render shortening of a diameter of each dot and stabilization of the dots and to increase resolution in the sub-scan direction. Accordingly, it is possible to form high-resolution images.
Further, a ratio of a static beam-spot diameter Wm in a main scan direction on the surface of the photosensitive body defined by 1/e2 of the maximum value in the exposure distribution of the beam spot to the static beam-spot diameter Ws in the sub-scan direction on the surface of the photosensitive body defined by 1/e2 of the maximum value in the exposure distribution of the beam spot satisfies the following formula:
Wm/Ws less than 1 
Thereby, it is possible to increase resolution in the main scan direction, and to form high-resolution images also high in resolution in the main scan direction.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.